Demand for (zinc) plated steel strips, which enhance corrosion resistance, etc., have desirable aesthetic qualities, and are particularly used as steel sheets for electronic products or automobiles, has rapidly increased, and requirements for the quality of (zinc) plated steel strips have also increased.
Although not shown in a separate drawing, a zinc plating process for steel strips is, for example, performed by allowing molten zinc to be attached to surfaces of a steel strip while the steel strip is unwound from a pay off reel and passes through a snout and a plating bath in an in situ process.
At this time, a gas wiping apparatus (for example, an air knife) provided directly above the plating bath sprays a gas (an inert gas or air) onto a surface of the steel strip to properly reduce the amount of zinc attached to the surface of the steel strip, thereby controlling the plating thickness of the steel strip.
At this time, since various types of steel strip may be passed through the plating bath, widths, thicknesses and loads applied to (a shaft of) a sink roll in the plating bath are different, depending on the type of the steel strip, while the steel strip that has passed through the sink roll and a stabilizing roll passes through an upper transfer roll, vibrations in the steel strip occur or a curvature (C-shaped curvature or S-shaped curvature) phenomenon in which the steel strip is curved occurs even though the vibrations and curvature may vary with the type, the width or the thickness of the steel strip.
The occurrence of vibrations or a curvature phenomenon may cause the interval between the gas wiping apparatus and the steel strip to be non-uniform, to thus generate a plating deviation, resulting in a plating failure.
Therefore, although not shown in a separate drawing, at least one steel strip stabilizing apparatus for suppressing vibrations in the steel strip or correcting the shape thereof is disposed between the gas wiping apparatus and the upper transfer roll.
For example, an existing steel strip stabilizing apparatus (“steel strip damping apparatus or shape correcting apparatus”) dampens (suppresses) the vibrations in the plated steel strip or removes the curvature thereof to thus correct the shape of the steel strip, thus preventing the occurrence of deviations in plating by using a mechanical touch roll contacting the steel strip or spraying a gas onto the steel strip.
However, in the case of using the mechanical touch roll, since the roll contact is performed in a state in which the zinc layer plated on the steel strip is completely unattached (dried), a secondary quality defect, such as a surface roll marker may be caused.
Therefore, a non-contact type steel strip stabilizing apparatus (damping mechanism) using electromagnetic force, which realizes vibration damping of a steel strip in a non-contact manner or corrects the shape of the steel strip has also been used recently.
However, as explained through FIG. 5A of the present invention, the existing steel strip stabilizing apparatus using electromagnetic force is problematic in that the positions of unit damping blocks (damping means) are fixed with respect to the steel strip in a widthwise direction thereof.
For example, in the case of the existing non-contact type steel strip stabilizing apparatus using electromagnetic force, since 5 to 7 damping units (electromagnet blocks) are fixedly disposed per side of the steel strip, the unit damping units have difficulty in controlling the position while moving, corresponding to various widths of the steel strips.
Therefore, when the width of a steel strip is changed, particularly when an edge of the steel strip is positioned in a space (gap) between the damping blocks, an unstable damping region (shape correcting region) is generated while a discontinuous boundary surface is formed.
That is, when an edge portion of the steel strip is positioned between the damping means in which a magnetic field is not formed, it is difficult to apply a uniform degree of electromagnetic force in the widthwise direction with regard to the steel strip, and problems, such as an increase in vibrations or instability in the shape of the steel strip may occur while the damping capability is reduced at the edge of the steel strip.
Therefore, in the case of the existing steel strip stabilizing apparatus, it is necessary to arrange as many unit damping units as possible in the widthwise direction with regard to the steel strip. In consideration of the maximum width of currently available plated steel strips, at least seven damping units are required to reduce an interval therebetween, such that the steel strip damping capability or the shape correcting capability is not reduced due to the formation of the discontinuous boundary surface explained above.
Therefore, since the existing steel strip stabilizing apparatus needs a lot of unit damping units, facility establishment costs or facility maintenance costs may be high.